There are several different ways of exciting a light-emitting semiconductor such that it emits light. Photoluminescence arises when the excitation takes place by illumination by light of a wavelength shorter than that of the emitted light. During electroluminescence the excitation takes place by injecting current into the semiconductor.
The radiance of a light source is defined as the light flux per m.sup.2 and per steradiane (w m.sup.-2 st.sup.-1). The radiance of a light-emitting semiconductor is proportional to the number of injected electron-hole pairs, the so-called charge carrier density.
In spite of the fact that the excitation inside the semiconductor is performed effectively, only a small part of the emitted light is released from the semiconductor. The reason for this is, on the one hand, that the semiconductor has a high refractive index and, on the other hand, that the emitted light is isotropic, that is, it is equally distributed in all directions. A high refractive index means that only that light which becomes incident within a narrow cone perpendicular to the surface of the semiconductor is released, whereas the remainder of the light is reflected in the surface of the semiconductor.
A good luminous efficiency in the semiconductor is obtained by giving it a double hetero (DH) structure, which is built up of three layers in which one thin layer of the light-emitting semiconductor, that so-called active layer, is surrounded by two thicker layers, so-called barrier layers, of another semiconductor material with a larger band gap than that of the active layer.
It is previously known that if a DH structure is placed in a microcavity, the emitted light can be directed. A microcavity comprises two mirrors and the resonance wavelength of the microcavity is determined by the distance between the mirrors. This means that when the emitted wavelength is exactly equal to the resonance wavelength of the microcavity, the light will be directed substantially perpendicularly to the surface of the semiconductor and hence be released from the semiconductor. Only a small part of the light is reflected in the surface of the semiconductor. As mirrors in the microcavity, Bragg reflectors can be used, which consist of a large number of layers of semiconductor material with alternately high and low refractive index. The microcavity can be excited by external radiation (photoluminescence) or by current (electroluminescence).
If the light-emitting layer is made sufficiently thin, a quantum well is formed and the emitted wavelength can be influenced by an external electric field, so-called Stark effect.
When the quantum well emits light with the same wavelength as the resonance wavelength of the microcavity, the light is directed and the radiance is increased. When the quantum well emits light, the wavelength of which deviates from the resonance wavelength of the microcavity, the radiance is reduced. In this way, it is possible to modulate the radiance by varying the electric field across the microcavity. The electric field can be modulated with a much higher frequency than that with which the charge carrier density in the semiconductor can be modulated.
If the quantum well is excited by injecting current via two electrodes on different sides of the microcavity, the electric field across the quantum well cannot be varied independently of the current, and the radiance cannot thus be modulated.
In European patent application with publication No. 0 473 983 A2, the above-mentioned problem has been solved by arranging, in addition to the two electrodes on different sides of the microcavity, a common electrode on the layer which contains the quantum well. This means that the quantum well is excited by the current which flows between the first electrode and the common electrode. The electric field across the quantum well is varied with the common electrode and the third electrode. The radiance can thus be modulated.
This way of solving the problem entails several disadvantages. The current must flow through a Bragg reflector, which has a very high resistance because of the many junctions between materials with different band gaps. Arranging the common electrode on the layer which constitutes the quantum well also entails difficulties since this layer is very thin (100 .ANG.).
To obtain the fastest possible modulation, the changeover time must be made as short as possible. The changeover time is proportional to the capacitance in the microcavity, which in turn is proportional to the area of the quantum well. With the above-mentioned solution, the area of the quantum well is determined by the size of the contact to the common electrode. This sets a lower limit to the changeover time.
The above-mentioned disadvantages can be eliminated by exciting the quantum well with light (photoluminescence) instead. When using an external radiation source, problems with poor efficiency instead arise. On the one hand, it is difficult to extract the radiation efficiently from the radiation source, and on the other hand it is difficult to get the radiation into the microcavity. Because of the high refractive index of the semiconductor materials, the efficiency from an external light source is only about 2%.
One object of the invention is to suggest a method and a device for emitting light which effectively uses light to excite a quantum well, which allows the radiance to be modulated with a very high frequency.